Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager October 1, 2008 Page 17 of 31 k I o t z associates Table 3 summarizes the type of studied channels for the first two above categories (see also Exhibit 11). Major drainage ditches or channels evaluated for the CWDS are of two types: 1) those for which FEMA-based hydrologic and hydraulic models are available as a basis for model creation, update and revision, and 2) those for which survey has been done to approximately define the cross sectional shape of the channel. The FEMA-based models are for extensive lengths of channel and for relatively large drainage areas. The level of detail in the geometric description of these models is good as evidenced by the number of cross sections used in the model (see Exhibits 13, 14 and 15). Existing hydrologic and hydraulic models as given by FEMA models for these major drainage ditches and channels were reviewed and modified to reflect new conditions (e.g.,; change in channel. connectivity, crossing structures, or land use) and information about the waterway and the runoff to the waterway. These models provide a definitive description of flooding conditions along the ditch or channel, and detailed conclusions about flow containment or bank overflow along the length of the channel can be drawn. On the other hand, those ditches and channels for which no prior hydrologic and hydraulic models are available are generally short tributaries to the FEMA-based models. Only limited data on ditch or channel geometry are available for these ditches and channels; these data were obtained by field survey done for this study (see Exhibits 13, 14, and 15). Typically only 2 or 3 cross sections were surveyed along the ditch or channel. These survey data were collected for this study since no existing data describing channel geometry for the tributaries were available. In addition, information about structures (e.g., bridges) along the length of the channel is generally incomplete. Consequently it is difficult to draw reliable conclusions about the detailed behavior of water levels and consequent flow containment within channel banks. (Along F101-00-00 for which a HEC-RAS model was developed from hard copy data in the appendices of a master drainage plan done by HCFCD for La Porte in 1987, two channel sections between Hwy 146 and Sens Road were field surveyed as a check on the cross section data in the 1987 master drainage plan study. Comparison of the survey cross section data to the cross data provided in the 1987 showed the section data to be reasonable consistent. Bank elevations and bank to bank distances were quite similar, as were channel shape and side slopes except near the channel bottom. Flow line elevations in the surveyed sections were about 4 ft deeper than the sections taken from the 1987 study.)